And I reckon it’s a genre effect. Pop/rock is American, so non-Americans sing like the Americans they’re emulating. John Williamson (singer), by contrast, does Australian Country with some overt Australian nationalism; and his vowels are, well, True Blue Aussie:

TISM were suffused to the gills with in-joke references to Australian culture. I got into them when I left Australia, out of nostalgia. I agree you wouldn’t mistake them for Americans, but those are not Australian vowels either, and there’s more rhotic action there than there should be. Because they were doing genre.

The two alt-rock bohemian self conscious hippie drongo skip bastard Aussie bands I loathed growing up were Frente! (with exclamation mark!) and Ratcat. The reason I loathed them, apart from being alt-rock bohemian self conscious hippie drongo skip bastards, was that they were among the first Australian acts to make a point of singing pop with an Australian accent: